1. Field of the Invention
The present invention relates to a controller for controlling rotational axes of a machine tool for machining gears, and in particular to a controller for machining gears by a gear generating motion between a workpiece, i.e. a gear blank and a tool having the shape of a gear and being in engagement with the workpiece.
2. Description of Related Art
A gear-generating method has been generally adopted for machining a gear by a gear-generating motion between a workpiece and a tool having the shape of a gear and meshing with a workpiece for machining a gear for gear-cutting and gear-grinding. There are known form cutting and the gear-generating using a hobbing cutter in the gear cutting and there are known polishing, shaving, lapping and honing in gear finishing.
In these gear machining, a driving method is known in which an axis for driving a gear blank (workpiece) and an axis for driving the tool are rotated in accordance with respective motion commands at respective velocities with a predetermined ratio in accordance with specifications of a gear to be generated and the tool, such as modules and the numbers of teeth of the gear and the tool. Another driving method is known in which a motion command for one of servo systems for driving the motors for driving the workpiece and the tool is determined to be a value obtained by multiplying a feedback signal from the other of the motors by a ratio predetermined in accordance with the specifications of the gear to be generated and the tool, so that the two axes are rotated in synchronism.
FIG. 6 is a block diagram of a control system constituted by a conventional controller in which the axes for the workpiece and the tool are driven in synchronism in accordance with respective motion commands. In this example, a tool axis to which a tool 1 is attached is driven by a first motor 15, and a workpiece axis to which a workpiece 2 is attached is driven by a second motor 25.
A servo system for the first motor 15 comprises a position control section 11, a velocity control section 12, a current control section 13 and a current amplifier 14. The first motor 15 is equipped with a position/velocity detector 17 for detecting position and velocity of the first motor 15 and outputs a position feed back amount PFB1 and a velocity feedback amount VFB1. A servo system for the second motor 25 comprises a position control section 21, a velocity control section 22 and a current control section 23 and a current amplifier 24. The second motor 25 is equipped with a position/velocity detector 27 for detecting position and velocity of the second motor 25 and outputs a position feed back amount PFB2 and a velocity feedback amount VFB2.
A position command issued from a host controller such as a numerical controller is directly inputted to the servo system for the first motor 15 for driving the tool 1. On the other hand, the position command issued from the host controller is multiplied by a ratio K in a multiplier term 3 and the obtained product is inputted to the servo system for the second motor 25 for driving the workpiece 2. The ratio K is predetermined in accordance with specifications of a tool and a gear to be formed, such as modules and the numbers of teeth of the tool and the gear.
The position control sections 11 and 21 perform position loop controls by multiplying position deviations between the position commands and the position feedback values PFB1 and PFB2 by position gains, respectively, to obtain velocity commands.
The velocity control sections 12 and 22 perform velocity loop controls such as proportional control and integral control based on velocity deviations between the velocity commands outputted from the position control sections 11 and 21 and the velocity feedback values VFB1 and VFB2, respectively, to obtain current commands.
The current control sections 13 and 23 perform current loop controls based on current deviations between the current commands outputted from the velocity control sections 12 and 22 and the current feedback values CFB1 and CFB2 form current sensors (not shown), respectively, to obtain voltage commands.
The current amplifiers 14 and 24 provide driving currents for the first and second motors 15 and 25 in accordance with the voltage commands outputted from the current control sections 13 and 23, respectively, to drive the first and second motors 15 and 25.
Since the position command for the second motor 25 has a value of the product of the position command for the first motor 15 and the ratio xe2x80x9cKxe2x80x9d, the second motor 25 is driven at a speed having the ratio K with respect to the speed of the first motor 15, to be synchronized with the rotation of the first motor 15, so that the workpiece 2 is driven at a speed having the ratio K with respect to the tool 1 synchronously with the tool 1.
In the example shown in FIG. 6, the position command from the host controller is directly into the servo system for the first motor 15 and the value obtained by multiplying the position command from the host controller by the ratio K is inputted to the servo system for the second motor 25 for driving the workpiece 2. Alternatively, the position command from the host controller is directly into the servo system for the second motor 25 and a value obtained by multiplying the position command by a predetermined ratio may be inputted into the servo system for the first motor 15.
The position control sections 11 and 21, the velocity control sections 12 and 22, and the current control sections 13 and 23 are constituted by digital servo processing by a processor of the controller.
FIG. 7 is a block diagram of a control system constituted by another conventional controller in which a feedback signal of one of the servo systems is multiplied by a predetermined ratio to obtain the position command for the other of the servo systems.
In the arrangement shown in FIG. 7, a position command from the host controller is inputted to the servo system for the first motor 15 for driving the tool 1, and the position feedback amount PFB1 from the position/velocity detector 17 of the fist motor 15 is multiplied by a ratio K and the obtained product is inputted as a position command for the servo system for the second motor 25 for driving the workpiece 2. The ratio K is predetermined in accordance with specifications of the tool and a gear to be generated such as module or the number of teeth of the tool and the gear. The arrangements and functions of the position control sections 11 and 21, the velocity control sections 12 and 22, the current control sections 13 and 23, the current amplifiers 14 and 24, and the position/velocity detectors 17 and 27 as shown in FIG. 7 are the same as those in the control system as shown FIG. 6.
In this example, the first motor for driving the tool 1 is driven based on the position command from the host controller and the second motor 25 is driven based on the position command obtained by multiplying the feedback amount of the first motor 15 by the ratio K, so that the second motor 25 is rotated to follow the rotation of the first motor 15, so that the tool 1 and the workpiece 2 are synchronously driven.
In the above described control systems by the conventional gear machining controllers, precision of machining of gears depends on characteristics of suppressing disturbance of the servo systems for controlling the axes of the tool 1 and the workpiece 2. Therefore, in the case of machining a gear of a large diameter, a disturbance torque (load torque) in machining increases to lower the precision of machining of the gear.
An object of the present invention is to provide a gear machining controller capable of performing a high-precision machining of a gear.
A gear machining controller of the present invention has servo systems for controlling a first motor for driving a gear machining tool and a second motor for driving a workpiece. The controller comprises: position control means provided in the servo systems for respectively controlling positions of the first motor and the second motor based on position deviations between position commands and position feedback amounts at every processing period; and position amending means provided for at least one of the position control means, for storing amendment amounts based on the position deviations for each cycle period, and amending the position command at a present processing period based on the amendment amount at the processing period preceding by one cycle period. Thereby, an error of synchronism of the first and second motors is reduced to improve precision of machining of gears.
The cycle period may be predetermined to be a period of cycle of a load pattern repeatedly present in driving the first and second motors. Specifically, the cycle period may be predetermined in accordance with a specification of a gear to be machined.
The position amending means may comprise a memory for storing data based on the position deviations for each cycle period, and a dynamic characteristic compensation element for outputting the data stored in the memory at every processing period.
The workpiece and the tool may be driven at a constant ratio based on specifications of the gear to be machined and the tool. One of the first and second motors may be driven in accordance with a position command obtained by multiplying a feedback value of the other of the first and second motors by a constant ratio based on specifications of the gear to be machined and the tool.